The proposed work will examine the hematopoietic microenvironment for its effect upon 1) lodgement, 2) commitment and 3) proliferation of the hematopoietic stem cells (pluripotent CFU) and their descendant cells in mice. Completion of studies on the first 3 days of development of spleen and femoral hematopoietic colonies will attempt to establish the precise site of CFU lodgement. Commitment and the possibility of its reversal will be studied in the colony types, especially erythrocytic/granulocytic, of bones and spleen of supralethally irradiated recipients of largely pluripotent CFU (from donor marrow) or largely unipotent CFU (committed to the erythropoietic line, as from donor fetal liver). The necessity of the hematopoietic micro-environment for proliferation among Friend virus transformed erythrocytic precursors will also be studied. BIBLOGRAPHIC REFERENCE: Wolf, N.S. and J.J. Trentin, Linearity of counts of endogenous and exogenous colonies in mouse bone marrow. Exp. Hemat. 3:54-56, 1975.